The invention relates to a scanning device including a transducer for scanning a recording medium which comprises parallel adjacent tracks, the transducer producing a scanning signal indicative of the recording medium properties at the positions of a scanning point thereon. The device comprises drive means for displacing the recording medium relative to a carrier body in the track direction, which carrier body is displaceable relative to the drive means in a direction transverse to the tracks. At least the portion of the transducer which determines the position of the scanning point on the recording medium is movable relative to the carrier body in a direction transverse to the tracks. The device also includes further drive means for displacing the carrier body in response to a drive control signal and an actuator for displacing the movable transducer portion relative to the carrier body in response to an actuator control signal. The device further includes tracking means for maintaining the scanning point on one of the tracks by adjustment of the actuator control signal in response to a tracking error signal, and displacement control means for controlling the displacement of the carrier body relative to the recording medium drive means in accordance with a specific displacement profile by adjustment of the carrier body drive control signal. The device is capable of adopting a tracking mode and a displacing mode, the tracking means being active in the tracking mode and the displacement control means being active in the displacing mode.
Such a device in the form of an optical read/write device with a two-stage controller for controlling the position of a scanning point caused by a scanning beam is known from EP-A 0 463 959. In the device described therein the carrier body is a mechanical slide. The movable transducer portion is a lens by which the scanning beam is focused on the recording medium. The lens can be moved relative to the slide by means of a so-termed focusing actuator which comprises a swivel arm on which the lens is installed. The position of the swivel arm can be changed by energizing the actuator coils. In the displacing mode, in which the scanning point is displaced to a new target track, the slide speed is controlled in accordance with a specific speed profile. The actuator control signal is maintained constant for virtually the entire time interval in which the slide is displaced. This achieves that, during the slide displacement, the actuator is maintained in a fixed position in the center of its displacement range. Therefore, when the device is to be returned to the tracking mode at the end of the displacing mode, the lens has a good starting position for locking on to the target track.
The prior-art focusing actuators are usually balanced, which is to say that the point of rotation of the swivel arm coincides with its center of gravity. This means that the position of the actuator relative to the slide is hardly influenced when the speed of the slide is increased, so that during the slide displacement the lens is kept in its place by means of a constant actuator control signal. Such a focusing actuator has a number of disadvantages. Firstly, the construction of such an actuator is relatively complex. Secondly, tolerance is nearly always present in such an actuator, which is detrimental to feedback control systems. Thirdly, the focusing actuator starts presenting residual magnetism at high speeds, so that focusing by adjustment of the distance from the lens to the medium by the focusing actuator is rendered more difficult. Other types of actuators, such as, for example, actuators acting on a spring suspension lens without making contact, do not have these detrimental effects. However, with high speeds or delays of the slide a constant actuator control signal will not be sufficient for the lens to be maintained in its fixed position and it will be pushed to an outermost position of its displacement range. Consequently, at the moment the new target track is to be locked on to, the lens will have an unfavorable starting position.